An improved relative permeability model for coal reservoirs

In this work, the conventional relative permeability model for two phase flow in porous media is improved to describe the relative permeability for coal. The fracture geometry is considered through applying the matchstick model, instead of the bundle of capillary tubes model which is often used as the conceptual model for conventional porous media, to derive the relative permeability model. The effect of porosity change on relative permeability for coal is taken into account by introducing a residual phase saturation model and a shape factor as functions of permeability ratio. In the improved model, the relative permeability is dependent on both the phase saturation and the porosity (or permeability) change. This improved model shows a strong capability to match the experimental data for different coal relative permeability measurements. Furthermore, we evaluate the relative permeability models as a unary function of wetting phase saturation and as a binary function of wetting phase saturation and permeability ratio in a coupled numerical model for water–gas flow in coal seams. The results illustrate that the relative permeability change due to the porosity change can significantly affect the evolution of wetting phase saturation and the gas production rate.

► A new relative permeability model for fractured and deformable coal reservoirs ► The new model is a function of wetting phase saturation and porosity change. ► The new model is able to represent well the experimental data from the literature. ► Porosity change on relative permeability may significantly affect the CBM recovery.